Figure 6.4. The subtractive color system uses cyan, magenta, and yellow colors.

In subtractive output devices such as color printers or printing presses, cyan, magenta, yellow, and, usually, black pigments (for detail) are used to represent a gamut of colors. It's obvious why additive colors won't work for hard copies: It is possible to produce red, green, and blue pigments, of course, and we could print red, green, and blue colors that way (that's exactly what is done for spot color). However, there would be no way to produce any of the other colors with the additive primaries. Red pigment reflects only red light; green pigment reflects only green. When they overlap, the red pigment absorbs the green, and the green absorbs the red, so no light is reflected and we see black.

This document is created with trial version of CHM2PDF Pilot 2.16.100. Cyan pigment, on the other hand, absorbs only red light (well, it is supposed to). It reflects both blue and green (theoretically), producing the blue-green shade we see as cyan. Yellow pigment absorbs only blue light, reflecting red and green, while magenta pigment absorbs only green, reflecting red and blue. When we overlap two of the subtractive primaries, some of at least one color still reflects. Magenta (red-blue) and yellow (red-green) together produce red, because the magenta pigment absorbs green and the yellow pigment absorbs blue. Their common color, red, is the only one remaining. Of course, each of the subtractive primaries can be present in various intensities or percentages, from 0 to 100%. The remainder is represented by white, which reflects all colors in equal amounts.

So, in our example above, if the magenta pigment was only 50% present and the yellow represented at 100%, only half of the green would be absorbed, while 100% of the blue would be soaked up. Our red would appear to be an intermediate color, orange. By varying the percentages of the subtractive primaries, we can produce a full range of colors.

Well, theoretically we could. You'll recall that RGB displays aren't perfect because the colors aren't pure. So, too, it is impossible to design pigments that reflect absolutely pure colors. Equal amounts of cyan, magenta, and yellow pigment should produce black. More often, what you'll get is a muddy brown. When daily newspapers first began their changeover to color printing in the 1970s, many of them used this three-color system, with mixed results.

However, better results can be obtained by adding black as a fourth color. Black can fill in areas that are supposed to be black and add detail to other areas of an image. While the fourth color does complicate the process a bit, the actual cost in applications like offset printing is minimal. Black ink is used to print text anyway, so there is no additional press run for black. Moreover, black ink is cheaper than critical process color inks, so it's possible to save money by using black instead of laying on three subtractive primaries extra thick. A typical image separated into its component colors for printing is shown in Figure 6.5.

Figure 6.5. Full-color images are separated into cyan, magenta, yellow, and black components for printing.

Figure 6.5. Full-color images are separated into cyan, magenta, yellow, and black components for printing.

The output systems you use to print hard copies of color images use the subtractive color system in one way or another. Most of them are unable to print varying percentages of each of the primary colors. Inkjet printers, color laser printers, and thermal wax transfer printers are examples of these. All these systems must simulate other colors by dithering, which is similar to the halftoning system discussed earlier. A few printers can vary the amount of pigment laid down over a broader range. Thermal dye sublimation printers are an example of this type. These printers can print a full range of tones, up to the 16.8 million colors possible with 24-bit systems.

The subtractive color system can also be represented in three dimensions, as I've done in Figure 6.6. In this illustration, the positions of the red, green, and blue colors have been replaced by cyan, magenta, and yellow, and the other hues rearranged accordingly. In fact, if you mentally rotate and reverse this figure, you'll see that it is otherwise identical to the one in Figure 6.3; RGB and CMYK are in this sense two sides of the same coin. However, don't make the mistake of thinking their color spaces are identical. There are colors that can be displayed in RGB that can't be printed using CMYK.

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